1. Field of the Invention
The present invention relates to a system and a method for position determination, and more particularly to a system and a method for determining the location of a roving mobile unit and providing location relevant information to the mobile unit.
2. Description of the Related Art
In recent years, commercial applications based on positioning systems have been developed. Location-relevant services, which provide information or perform services based on the geographical location of a mobile client, are examples of such positioning system applications. A location-relevant service can be used to provide travel-related services (e.g., driving directions) based on the position of the mobile client. One example of a location-relevant service is described, for example, in copending and commonly assigned U.S. patent application Ser. No. 09/422,116, entitled “Method for Distribution of Locality-Relevant Information using a Network” of Rodric C. Fan, filed Oct. 20, 1999 (“the '116 application”). To provide an example regarding the architecture and application of a location-relevant information system, the disclosure of the '116 application is incorporated herein by reference in its entirety.
Typically, a location-relevant system includes a location-relevant service server accessible by multiple mobile units over a communication system, which includes wireless links to the mobile units. The mobile units provide their positions based on a positioning system. The position of a mobile unit can be provided, for example, by the Global Positioning System (GPS) or, in a cellular telephone network, by a process called triangulation which is based on signal delays from system base stations of known fixed locations. Due to technology limitations at the present time, it is costly to integrate the capabilities of both positioning and wireless communication of such a mobile unit into a hand-held device, such as a mobile telephone or a personal digital assistant (PDA).
Even if a hand-held device is equipped with a positioning system, inherent shortcomings of existing positioning system technology limit the hand-held device's ability to determine its position solely based on the positioning system. Often, there are geographical areas in which the positioning system does not function, such as when the positioning system is not able to acquire the necessary positioning signals. For example, when GPS is used for position determination, a GPS receiver must be able to observe four or more GPS satellites in orbit around the earth and acquire information from the GPS satellites. Because a GPS receiver requires an unobstructed view of the GPS satellites, the GPS receiver typically does not work well indoors where the satellite signals may be obstructed or weak, or in urban areas where buildings often obstruct the view of the GPS satellites. On the other hand, when position determination using triangulation in a cellular telephone network is used, the hand-held device must be able to obtain signals from three surrounding base stations. Thus, such a positioning system relies on the availability of cellular base stations in the area and typically has limited service range.
Nonetheless, there is a great demand in incorporating position determination capability in a wireless mobile unit, particularly in a hand-held device, to enable a user of the mobile unit to conveniently obtain location-relevant information. Furthermore, recent Federal Communications Commission (FCC) mandate for enhanced 911 emergency services (E911) requires a wireless telephone to be capable of providing location information for 911 calls. Thus, position determination in wireless telephones is becoming a necessity in order to meet the requirements of the E911 service.
A position determination method using the United States zip code has been described. To determine a user's current location, the user of a mobile unit keys in the 5-digit United States zip code of the area the user is currently located. The location-relevant service uses the zip code to determine the current location of the mobile unit. This method has several shortcomings. First, the position determination is not accurate because the method only determines in which geographical area covered by that zip code the mobile unit is located. The geographical area covered by a zip code can be very large. Thus, the zip code based position determination can only provide a gross estimate of the mobile unit's location. Second, the zip code based position determination method requires the user of a mobile unit to know the zip code of her current location. However, when the user is lost or traveling in an unfamiliar area, it is not likely that the user would know the zip code of her current location. Therefore, the zip code based position determination method is not practical in operation.
Therefore, it is desirable to provide means for obtaining location information of mobile units, particularly in areas where the existing positioning systems cannot provide adequate service.